September drilling increases the risk of BYDV because plants emerge during the peak of the flight period of aphid vectors. The trend in recent seasons is that milder autumn temperatures and fewer frosts have extended the period of aphid activity, which not only increases the risk of initial infection, but also increases the threat from secondary spread within the crop.
Yield losses in barley and oats can be as high as 70%, wheat is less severely affected with losses up to 30% and both triticale and rye can also be infected. The earlier in the plants development the infection occurs, the greater the impact on yield.
The initial infection occurs following the colonisation of relatively low numbers of plants within the crop. It is the spread of the second generation of aphids within the crop that gives rise to the greatest levels of infection, and it is the control of this secondary spread which is the key to a successful BYDV management strategy.
Crops may require treatment with aphicide within 2-3 weeks of emergence and an application should be made if aphids are found in the crop from the 2-3 leaf stage. Secondary spread occurs when the accumulated daily temperature above a base of 3oC, reaches a T-Sum of 170. The T-Sum calculation should start from the date of crop emergence - if an aphicide is applied the T-Sum is re-set to zero and the calculation begins again 7 days after the crop is sprayed.
There is known to be some resistance to pyrethroid insecticides in aphid populations. As part of an anti-resistance strategy, at risk crops should only be treated once aphids are present and the label rate recommendations should be followed. The efficacy of pyrethroid treatments should be monitored, a second application of a pyrethroid product should not be made if control is poor.
There are a small number of varieties of both wheat and barley which have tolerance to BYDV and when planning ahead, they could form part of an integrated management plan to reduce yield losses.
